Peri-Dimethylamino substituent effects on proton transfer at carbon in α-naphthylacetate esters: A model for mandelate racemase

Article abstract of DOI:10.1039/c1ob06525d

The rate constants for exchange of hydrogen for deuterium at the α-CH₂ positions of 8-(N,N-dimethylaminonaphthalen-1-yl)acetic acid tert-butyl ester 1 and naphthalen-1-ylacetic acid tert-butyl ester 2 have been determined in potassium deuteroxide solutions in 1:1 D₂O:CD ₃CN, in order to quantify the effect of the neighbouring peri-dimethylamino substituent on α-deprotonation. Intramolecular general base catalysis by the (weakly basic) neighbouring group was not detected. Second-order rate constants, k_DO, for the deuterium exchange reactions of esters 1 and 2 have been determined as 1.35 × 10^-4 M^-1 s^-1 and 3.95 × 10^-3 M^-1 s^-1, respectively. The unexpected 29-fold decrease in the k _DO value upon the introduction of a peri-dimethylamino group is attributed to an unfavourable steric and/or electronic substituent effect on intermolecular deprotonation by deuteroxide ion. From the experimental k _DO values, carbon acid pK_a values of 26.8 and 23.1 have been calculated for esters 1 and 2.

Full text of DOI:10.1039/c1ob06525d

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PAPER
peri-Dimethylamino substituent effects on proton transfer at carbon in
a-naphthylacetate esters: a model for mandelate racemase†
Richard J. Delley,a Subhajit Bandyopadhyay,b,c Mark A. Fox,a Constanze Schliehe,d David R. W. Hodgson,a
Florian Hollfelder,*b Anthony J. Kirby*e and AnnMarie C. O’Donoghue*a,d
Received 5th September 2011, Accepted 11th October 2011
DOI: 10.1039/c1ob06525d
The rate constants for exchange of hydrogen for deuterium at the a-CH2 positions of
8-(N,N-dimethylaminonaphthalen-1-yl)acetic acid tert-butyl ester 1 and naphthalen-1-ylacetic acid
tert-butyl ester 2 have been determined in potassium deuteroxide solutions in 1 : 1 D2O : CD3CN, in
order to quantify the effect of the neighbouring peri-dimethylamino substituent on a-deprotonation.
Intramolecular general base catalysis by the (weakly basic) neighbouring group was not detected.
Second-order rate constants, kDO, for the deuterium exchange reactions of esters 1 and 2 have been
determined as 1.35 ¥ 10-4 M-1 s-1 and 3.95 ¥ 10-3 M-1 s-1, respectively. The unexpected 29-fold decrease
in the kDO value upon the introduction of a peri-dimethylamino group is attributed to an unfavourable
steric and/or electronic substituent effect on intermolecular deprotonation by deuteroxide ion. From the
experimental kDO values, carbon acid pKa values of 26.8 and 23.1 have been calculated for esters 1 and 2.
from carbon as a common part of more complex, diverse reaction
schemes.6–8
Introduction
Proton transfers are arguably the most abundant and important
processes in biology. Although normally fast, they can become rate
determining, and thus need efficient catalysis. This holds especially
when the proton is transferred to or from carbon, or when the
reaction is concerted with the formation or cleavage of a bond
between heavy (i.e. non-hydrogen) atoms. We are interested in the
factors that control proton transfer in solution and at enzyme
active sites. Small molecule models that incorporate catalytic and
We present a synthetic small molecule model 1 designed
substrate functional groups provide an opportunity to study the
to mimic the enzyme–substrate complex in MR by juxtapos-
chemistry in an enzyme–substrate complex, in which functional
ing a naphthylamino group peri to a benzylic C–H bond.
groups are brought together by an enzyme.1–5 Mandelate racemase
This geometric arrangement had been shown to be unusu-
(MR) catalyses a proton transfer reaction, namely the abstraction
ally efficient for the reverse proton transfer reaction in enol
of a substrate benzylic proton from carbon to an enzyme amine
ether hydrolysis catalysed by a peri-dimethylammonium group
base, Lys-166. MR represents a broader enzyme superfamily that
(Scheme 1).9
has been postulated to have evolved to catalyze proton abstraction
aDepartment of Chemistry, University of Durham, South Road, Durham,
UK, DH1 3LE. E-mail: annmarie.odonoghue@durham.ac.uk; Tel: +44 (0)
191 334 2592
bDepartment of Biochemistry, 80 Tennis Court Road, Cambridge, UK, CB2
1GA. E-mail: fh111@cam.ac.uk; Tel: +44 1223 766048
cCurrent address: Indian Institute of Science, Education and Research -
Kolkata, Mohanpur, Nadia, West-Bengal, 741252, India. E-mail: sb1@
Scheme 1 peri-Dimethylammonium catalysis of enol ether protonation.
iiserkol.ac.in
dThe School of Chemistry and Chemical Biology, University College Dublin,
Belfield, Dublin 4, Ireland
The efficiency of catalysis can be measured by the effective
molarity (EM), defined as the ratio of intramolecular first-order
and intermolecular second-order rate constants. Small molecule
models of intramolecular proton transfer exhibit low EMs unless
eDepartment of Chemistry, Lensfield Road, Cambridge, UK, CB2 1EW.
E-mail: ajk1@cam.ac.uk; Tel: +44 1223 336370
† Electronic supplementary information (ESI) available: all kinetic data
included. See DOI: 10.1039/c1ob06525d
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the product, and the transition state that leads to it, are stabilized
by a strong intramolecular hydrogen bond.3,9–12
Fig. 1 shows representative partial 1H NMR spectra of a-
naphthyl ester 2 obtained during exchange of the a-protons
for deuterium in the presence of potassium deuteroxide in 1 : 1
D2O : CD3CN at 25 ◦C (see ESI for analogous NMR spectra
for ester 1†). The exchange of hydrogen for deuterium led
to disappearance of the singlets at 4.71 and 4.49 ppm, and
appearance of upfield multiplets at ~ 4.69 and 4.47 ppm for esters
1 and 2, respectively. The upfield multiplets correspond to the
a-CHD groups of monodeuteriated a-naphthyl esters 7 and 8
(Scheme 3).
Two examples serve to illustrate this point. Based on the same
geometric arrangement as in 1, the ketonization reaction of enol
ether 3 (Scheme 1) is catalyzed extraordinarily efficiently by the
neighbouring peri-dimethylammonium group with an EM above
60 000.9 Its efficiency may be rationalized by the formation of
a strong intramolecular hydrogen bond in oxocarbenium ion 4.
A smaller, but still substantial, EM of 2000 was determined for
catalysis by the ortho-carboxyl group of the cyclization of enol
ether 5 to acylal 6 (Scheme 2).12 This efficient general acid catalysis
was also attributed to the stabilisation of the transition state for
proton transfer by intramolecular hydrogen bonding.
Scheme 2 Intramolecular catalysis of enol ether protonation.
These EM values for intramolecular processes involving proton
transfer at carbon are the highest reported: the runners-up
typically weigh in around 1 M or less.
We report investigations of proton transfer at the a-CH2
positions of 8-(N,N-dimethylaminonaphthalen-1-yl)acetic acid
tert-butyl ester 1 and, for comparison, of naphthalen-1-ylacetic
acid tert-butyl ester 2 that is lacking the intramolecular catalytic
1
group. We have employed H NMR spectroscopy to follow the
exchange of hydrogen for deuterium at the a-CH2 positions of
a-naphthyl esters 1 and 2 in order to probe the effect of the peri-
dimethylamino substituent on keto–enol tautomerization.
Results
Deuterium exchange experiments
The exchange for deuterium of the a-protons of a-naphthyl esters
1 and 2 (Scheme 3) was followed by monitoring the disappearance
of the a-CH2 groups of the substrates by 1H NMR spectroscopy.
Owing to the poor solubilities of both substrates in water, an
acetonitrile co-solvent was required. Competing hydrolyses of
esters 1 and 2 to the corresponding naphthyl acetic acids were
not observed because of the choice of sterically hindered tert-
butyl esters as substrates. It was therefore possible to follow the
deuterium exchange reaction of both esters for three half-lives in
the absence of any significant side reactions.
Fig. 1 Representative partial 1H NMR spectra at 400 MHz of ester 2
obtained during exchange of the a-protons for deuterium in the presence
of KOD in 1 : 1 D2O : CD3CN at 25 ◦C and I = 0.1 M (KCl). The percentage
of the unexchanged a-CH2 group of the ester substrate remaining in each
sample is indicated above each spectrum.
The observed pseudo-first-order rate constants for exchange
of the a-protons of esters 1 and 2 for deuterium, kobs, were
obtained from the slopes of semilogarithmic plots of reaction
progress against time according to eqn (1) (all plots are included
in the ESI†). The values of f (s), the fraction of unexchanged
substrate, were calculated from eqn (2), where ACH and Astd are
2
the integrated areas of the signals corresponding to the a-CH2
group of the ester and the twelve methyl hydrogens of the internal
standard tetramethylammmonium deuteriosulfate, respectively.
Table 1 gives the values of kobs (s-1), which were obtained at different
concentrations of potassium deuteroxide.
Scheme 3 Deuterium exchange at the a-carbon.
ln f (s) = -kobs
t
(1)
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Table 1 First and second-order rate constants for the exchange of the
a-protons of naphthyl esters 1 and 2 for deuterium in KOD solutions in
1 : 1 D2O : CD3CNa
only the deuteroxide-catalysed reactions are significant. Second-
order rate constants, kDO, of 1.35 ¥ 10-4 M-1 s-1and 3.95 ¥ 10-3 M-1
s-1 were obtained for the deuterium exchange reactions of esters 1
and 2, respectively.
Compound
[KOD]/M
k
obs/s-1b
k
DO/M-1 s-1c
kobs = kDO[DO-]
(3)
1
0.023
0.046
0.060
0.070
0.083
0.092
2.76 ¥ 10-6
5.69 ¥ 10-6
7.90 ¥ 10-6
9.30 ¥ 10-6
1.08 ¥ 10-5
1.18 ¥ 10-5
1.34 ¥ 10-4
Determination of the pKa of the peri-dimethylammonium
substituent
UV-Visible spectra of ester 1 were acquired in 1 : 1 H2O : CH3CN
solutions at a range of pH values. A comparison of spectra
obtained in 0.1 M HCl and 0.1 M KOH solutions showed
a distinct redshift upon deprotonation of the peri-substituent.
Spectra acquired in buffered solutions at pH values greater than
2.5 were identical to those obtained in 0.1 M KOH, which shows
that the substituent was present as the amino free-base form under
these conditions. Fig. 3 shows a range of UV-Vis spectra of ester
1 at different pH values. Non-linear least squares fitting to eqn
(4) of the changes in absorbance with pH at the chosen analytical
wavelength, l = 325 nm, gives Ka = 9.61 ¥ 10-2 M (pKa = 1.02
0.03) for the peri-dimethylammonium substituent (plot in the
ESI†). In eqn (4), Aobs is the observed absorbance at a given pH,
Amax is the absorbance of the neutral amino substituent and Amin
is the absorbance of the fully protonated ammonium ion form at
l = 325 nm.
2
0.023
0.046
0.060
0.070
0.083
0.092
8.28 ¥ 10-5
1.77 ¥ 10-4
2.35 ¥ 10-4
2.73 ¥ 10-4
3.20 ¥ 10-4
3.54 ¥ 10-4
3.95 ¥ 10-3
a At 25 ◦C and I = 0.1 M (KCl). b First-order rate constants for
deuterium exchange determined from plots of reaction progress against
time according to eqn (1). c Second-order rate constants for deuteroxide-
catalyzed exchange obtained from the slope of a plot of kobs against [KOD]
according to eqn (3).
(ACH / Astd
)
)
t
2
f (s) =
(2)
(ACH / Astd
0
2
The observed dependence of values of kobs (s-1) on KOD
concentration could be described by eqn (3). Plots of kobs values
against KOD concentration were linear (Fig. 2) with slopes
equivalent to kDO (M-1 s-1), the second-order rate constants for
deuteroxide-catalyzed exchange. The zero intercepts indicate that
Amin10−pH + KaAmax
(4)
Aobs
=
10−pH + Ka
Fig. 3 UV-Visible spectra of ester 1 (0.3 mM) in 1 : 1 H2O : CH3CN
solutions at a range of pH values.
Discussion
Proton transfer to carbon from the peri-dimethylammonium
group in 3 is so efficient (EM > 60 000) that the rate-determining
step is the opening of the strong intramolecular hydrogen bond to
the oxocarbenium ion intermediate 4 (instead of proton transfer)
followed by the rapid addition of the peri-dimethylamino group
(Scheme 1).9
Fig. 2 Plots of first-order rate constants, kobs, for exchange of the
a-protons of naphthyl esters 1 (ꢀ) and 2 ( ) for deuterium, against the
᭹
concentration of potassium deuteroxide (M) in 1 : 1 D2O : CD3CN at 25 ◦C
The ester 1 sets up the same geometry as system 4, with the
dimethylamino group in position vis a´ vis the a-protons of the
and I = 0.1 M (KCl).
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ester group to act as an efficient general base. However, only
deuteroxide-catalysed exchange is observed. We have determined
requirement for efficient proton transfer of a strong hydrogen bond
is not fulfilled. The pKa of the migrating a-proton of 4, estimated
as less than -3,12 could match that of the dimethylamino group
in the transition state.12 The much greater difference between the
pKas of the dimethylammonium group (1.02) and the carbon acid
pKa of ester 1 (26.8, see below) means that similar pKa matching
as the transition state is approached, as invoked as the condition
for the efficiency of mandelate racemase (and other enzymes),
cannot be achieved.14–18 Furthermore, there is a difference in
charge type between the hydrogen-bonded intermediate 4 and
that which could form between reactant 1 and enolate 11. In
intermediate 4, a partial positive charge will exist on both oxygen
and nitrogen atoms at the extremes of the hydrogen bond. In the
case of system 1, there would be a partial positive charge on the
terminal nitrogen of the bond but a (small) negative charge on
the a-carbon of the enolate, which will alter the hydrogen bond
strength.
Scheme 4 shows the possible pathways of exchange of hydrogen
for deuterium at the a-CH2 groups of esters 1 and 2. Formation of
the mono-deuteriated product 8 (bottom right in Scheme 4) from
2 can only occur by intermolecular deprotonation by deuteroxide
ion to yield an [enolate·HOD] complex 9. The replacement of the
initially formed HOD molecule by one of bulk solvent, present
in large excess, followed by rapid deuteration by D2O, then gives
initial exchange product 8.
a pKa value of 1.02
0.03 for the peri-dimethylammonium
group of naphthalene ester 1 in 1 : 1 H2O : CH3CN. Under our
experimental conditions (pD > 12), this peri-substituent will be
present in the neutral amino-form and any contribution from
an intramolecular reaction will be pD-independent. The absence
of a significant positive intercept on the ordinate of the second-
order plot for ester 1 (Fig. 2) implies that intramolecular general
base catalysis by the dimethylamino group is not competitive
with the intermolecular reaction of deuteroxide ion present at
concentrations of 0.025–0.092 M. Reactions at lower pD values,
or lower deuteroxide concentrations, were too ◦slow for practical
measurements by 1H NMR spectroscopy at 25 C.13 The effect of
the peri-dimethylamino group on the second-order rate constant
for deuteroxide-catalyzed exchange at the a-CH2 position of
naphthyl ester 1 is relatively small. The introduction of a peri-
dimethylamino group suppresses the kDO value by 29.3-fold rather
than increasing this value.
Intramolecular protonation of enol ether 3 by the peri-
dimethylammonium group (pKa = 4.01), does efficiently outcom-
pete intermolecular protonation by up to 1 M hydronium ion,
H3O+ (pKa = -1.74).9 By contrast, intramolecular deprotonation
of the a-CH2 group of ester 1 by the dimethylamino group
(pKa = 1.02, significantly less basic than might be expected from
3) does not compete with intermolecular deprotonation by the
more basic deuteroxide ion (pKa (D2O) = 16.6) present at 0.025–
0.092 M concentrations. Even though the geometry of ester 1
closely matches that of enol ether system 3, the complementary
The formation of mono-deuteriated product 7 from ester 1 by
an analogous intermolecular pathway will occur via enolate 10.
Additional routes to exchange product 7 involving intramolecular
deprotonation by the peri-dimethylamino group are also shown
Scheme 4 Potential pathways for deuterium exchange at the a-carbons of naphthyl esters 1 and 2.
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in Scheme 4. Deprotonation of the a-CH2 proton of 1 by the
peri-dimethylamino group would generate zwitterionic enolate
11. Subsequent incorporation of deuterium into enolate 11 could
occur by two possible routes. The exchange of the hydrogen of
the ammonium group of 11 for deuterium from bulk solvent D2O
would give enolate 12, and subsequent intramolecular transfer of
deuterium to the enolate would yield exchange product 7. Alter-
natively, enolate 11 could accept a deuterium atom at a-carbon
from a molecule of solvent D2O followed by the deprotonation of
the peri-dimethylammonium group.
tetrahedral pair of) methyl gr◦oups of 1,8-dimethylnaphthalene are
both splayed out by almost 5 from the trigonal geometry,29 while
systems with larger tetrahedral groups relieve peri-strain by ring
distortion.30
Based on the observed kDO values for intermolecular
deuteroxide-catalyzed exchange, carbon acid pKa values may be
estimated for esters 1 and 2. Using a secondary solvent deuterium
isotope effect of kDO/kHO = 1.419 and the experimental kDO values,
second-order rate constants for deprotonation of esters 1 and 2
by hydroxide ion at a-carbon can be calculated as kHO = 9.64 ¥
10-5 M-1 s-1 and 2.82 ¥ 10-3 M-1 s-1, respectively. J. P. Richard
and co-workers have constructed a correlation of kHO values for
deprotonation of neutral a-carbonyl acids with corresponding
carbon acid pKa values in water.20 An excellent linear relationship
has been observed for a broad range of aldehydes, ketones,
esters and amides using eqn (5), in which both kHO and pKa are
statistically corrected for the number of acidic protons, p, at the
carbon acid.
If intramolecular deprotonation were to occur, the thermody-
namically strongly favourable reprotonation of enolate 11 by the
acidic ammonium group would be expected to be too fast to permit
exchange to occur by either of the two routes outlined above.
Intermolecular protonation of the enolate of ethyl acetate (pKa =
25.6) by the conjugate acid ammonium ion of 3-quinuclidinone
(pKa = 7.5) has been estimated to be diffusion limited.19,20 The
intramolecular protonation of enolate 11, by the 6 pKa unit more
acidic peri-dimethylammonium ion should be significantly faster.
Owing to the proximity of the proton, this reprotonation reaction
will likely outcompete the diffusion of bulk solvent to and from
enolate 11, which is necessary for exchange to occur following
intramolecular deprotonation (Scheme 4). Furthermore, an in-
tramolecular hydrogen bond involving the migrating proton would
also hinder the external deuterium exchange. Hydrogen bonds to
weakly electronegative carbanions are thought to be weak, but
this could be an exception.21 The related steric barrier to rotation
of the NHMe2 group into a position to enable hydron exchange
with solvent further favours intramolecular reprotonation over
exchange in the case of enolate 11. A possible solution to increasing
the efficiency of intramolecular general base catalysis of exchange
might be attained via the availability of additional deuterons at
nitrogen, as for primary or secondary ammonium analogues to
enolate 11. This could provide a route to deuterium exchange that
does not require full diffusional separation, and might facilitate
intramolecular general base catalysis of exchange. A potential
drawback would be the competing lactonization reaction of the
peri-nitrogen with the ester group.
k
log( HO ) = 6.52−0.40(pKa + log p)
(5)
p
Using the kHO values calculated above, and p = 2, pKa values
of 26.8 and 23.1 can be estimated for esters 1 and 2, respectively,
which are similar to the value of 25.6 determined for ethylacetate
in aqueous solution.19 The 2.5 unit decrease in pKa observed in
comparing ethyl acetate and ester 2 is similar to the pKa decrease
that occurs upon introduction of an a-phenyl substituent to ethyl
acetate: the pKa for a-phenylethyl acetate in water is 22.7,31
compared to 25.6 in ethylacetate. This suggests that the effects
of the 50% acetonitrile co-solvent and the tert-butyl functional
group on carbon acidity are small relative to that of the a-
aryl substituent. The estimated pKa value for peri-substituted
ester 1 is 1.2 units higher than for ethyl acetate, thus the peri-
dimethylamino substituent counteracts the acidifying a-naphthyl
substituent effect resulting in an overall decrease in acidity relative
to the simple alkyl ester, ethyl acetate.
Implications
On the basis of the above discussion, we conclude that the
deuterium exchange reactions of esters 1 and 2 both occur
by intermolecular routes. The observed 29.3-fold suppression
of deuterium exchange by the peri-dimethylamino substituent
must be due to a steric and/or an electronic substituent effect
on intermolecular deprotonation by deuteroxide ion. Electronic
and steric effects in peri-disubstituted systems have been well-
documented.22–28 X-ray crystal structures of naphthalenes bearing
a dimethylamino group and an electron deficient carbonyl sub-
stituent in the peri-positions show that the pyramidyl dimethy-
lamino group is oriented with its lone pair in the space between
the two peri-groups as a strategy towards the relief of steric
strain.27 The presence of a lone pair of electrons on the peri-
nitrogen atom will disfavour the approach of deuteroxide ion
and the subsequent formation of the negatively charged enolate
intermediate. In addition, the steric bulk of this peri-substituent
could hinder the approach of deuteroxide ion to the a-CH2 group
of ester 1. The effective bulk of the peri-dimethylamino group will
increase on protonation, accounting for its significantly reduced
basicity in terms of strain in the conjugate acid in comparison
with the analogous group in enol ether 3. Thus the (smallest
Juxtaposition of functional groups is widely accepted as a source
of enzymatic catalysis. Evidence from two previous enzyme
models9,12 predicted that the geometry for proton abstraction in
model 1 would be optimal. Though this may indeed be the case,
quantification of any effect is impossible because intramolecular
reprotonation of the enolate intermediate is faster than the
external exchange reaction.32 An enzyme such as MR can avoid
this scenario by using its flexibility to make the reverse reaction less
efficient, e.g. by pulling the protonated amine group away from the
reaction centre. These observations highlight a limitation of simple
synthetic enzyme models that cannot easily mimic the inherent
ability of proteins to effect subtle conformational adjustments as
part of catalysis.
Experimental
Materials
Deuterium oxide (99.9% D) was purchased from Apollo Sci-
entific Ltd. Deuterium chloride (35% wt, 99.5% D), potassium
594 | Org. Biomol. Chem., 2012, 10, 590–596
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deuteroxide (40% wt, 98+% D), deuterated chloroform
(99.8% D), 1-naphthylacetic acid (>90%), and N,N-dimethyl-
1-naphthylamine (99%) were purchased from Sigma Aldrich.
N,N-Dimethyl-1-naphthylamine 13 and 1-naphthylacetic acid
14 (>90%), (99%) were recrystallized from ethanol. All other
chemicals were reagent grade and used without further purifi-
cation. Stock solutions of deuterium chloride and potassium
deuteroxide were prepared by dilution of commercial concentrated
standards, and titration against volumetric NaOH or HCl using
phenolphthalein as indicator.
11.5 mmol) in anhydrous THF (50 mL). The resulting solution
was stirred under argon for 24 h at room temperature. Pentane
(150 mL) was added to the mixture, and the solution was filtered
to remove the pyridinium chloride salt. The filtrate was washed
with saturated sodium chloride solution (2 ¥ 50 mL), saturated
sodium bicarbonate solution (2 ¥ 50 mL), and deionised water
(2 ¥ 50 mL). The solution was then dried over magnesium sulfate,
the solvent was removed in vacuo, and subsequent purification by
preparative TLC (cyclohexane : diethyl ether = 9 : 1) gave ester 1
(1.20 g, 4.9 mmol, 43%) as a colourless oil (found: C, 79.42; H, 7.47.
Calc. for C16H18O2: C, 79.31; H, 7.49%); Rf (cyclohexane : diethyl
ether = 9 : 1) = 0.80; nmax (neat)/cm-1 1726 and 1135; dH (700
MHz; CDCl3) 7.99 (d, 1H, J 8.5, aromatic-H), 7.85 (d, 1H, J
8.1, aromatic-H), 7.77 (d, 1H, J 11.5, aromatic-H), 7.49 (m, 2H,
aromatic-H), 7.40 (m, 2H, aromatic-H), 3.97 (s, 2H, ArCH2CO),
1.42 (s, 9H, C(CH3)3); dC (175 MHz; CDCl3) 171.2, 133.8, 132.2,
131.3, 128.7, 128.6, 127.8, 126.1, 125.5, 125.4, 123.9, 81.2, 40.5,
and 28.9; m/z (ESI+) 265.1 (100); HRMS (ESI+) C16H18O2Na
requires 265.1204, found 265.1207 (+1.1 ppm).
Synthesis
8-(N,N-Dimethylaminonaphthalen-1-yl)acetic acid tert-butyl es-
ter 1. n-Butyllithium (ca. 1.6 M, 10.5 mL in hexane, 16.8 mmol)
was added to a stirred solution of N,N-dimethyl-1-naphthylamine
13 (2.0 mL, 11.2 mmol) in anhydrous diethyl ether (18.0 mL) un-
der an argon atmosphere. N,N,N¢,N¢-tetramethylethylenediamine
(1.94 g, 16.8 mmol) was added to the solution after 15 min of
stirring, and the solution was stirred for an additional 14 h. The
reaction mixture was then cooled to -78 ◦C, and a suspension of
copper(I) cyanide (1.00 g, 11.2 mmol) in anhydrous diethyl ether
(15 mL) was added. Following the addition of CuCN, the reaction
flask was covered in aluminium foil in order to exclude light, and
the mixture was stirred for 1 h at◦room temperature. The reaction
mixture was then cooled to -78 C, and tert-butyl bromoacetate
(2.18 g, 11.2 mmol) was added. The flask contents were allowed to
warm to room temperature and stirred for 1 h. Methanol (150 mL)
and diethyl ether (100 mL) were then added to the reaction flask.
The resulting mixture was washed with water (4 ¥ 100 mL) and
dried over magnesium sulfate. The crude mixture was concentrated
in vacuo and purified by column chromatography (cyclohexane to
9 : 1 cyclohexane : diethyl ether) to give ester 1 as a colourless oil
(1.12 g, 3.91 mmol, 35%). Rf (cyclohexane : diethyl ether = 9 : 1)
= 0.38; dH (700 MHz; CDCl3) 7.75 (d, 1H, J 7.2, aromatic-H),
7.59 (d, 1H, J 7.2, aromatic-H), 7.40–7.36 (m, 2H, aromatic-H),
7.27 (d, 1H, J 6.3, aromatic-H), 7.23 (d, 1H, J 6.3, aromatic-
H), 4.34 (s, 2H, ArCH2CO), 2.73 (s, 6H, N(CH3)2), 1.42 (s, 9H,
C(CH3)3). dC (175 MHz; CDCl3) 172.0, 152.4, 136.6, 131.5, 131.1,
129.2, 129.0, 125.8, 125.7, 125.5, 117.9, 80.2, 46.6, 44.2, and 28.7;
m/z (ESI+) 308.2 (100); HRMS (ESI+) C18H23NO2Na requires
308.1626, found 308.1613 (-4.2 ppm).
Kinetic methods
Rate constants for exchange for deuterium of the first a-proton
of each of the naphthalene esters 1 and 2 in 1 : 1 D2O : CD3CN
were determined by monitoring the disappearance of the singlet
1
corresponding to the a-CH2 group of the substrate by H NMR
spectroscopy. Generally, the reactions of each substrate were
followed for at least three half-lives. All reactions were carried
◦
out in 1 : 1 D2O : CD3CN at 25 C and a constant ionic strength
(I) of 0.1 M maintained with potassium chloride.
In the case of 8-(N,N-dimethylaminonaphthalen-1-yl)acetic
acid tert-butyl ester 1, the progress of isotope exchange was
followed directly in the probe of the NMR spectrometer. Reactions
in a volume of 800 mL were initiated by the addition of 400 mL
of a stock solution of 1 (10 mM in CD3CN) to a solution
of potassium deuteroxide in D2O (400 mL), containing internal
standard, tetramethylammonium deuteriosulfate. NMR samples
(750 mL of above solution) were run at 25 ◦C in a Varian Mercury
500 MHz NMR spectrometer, in which spectra were continuously
obtained until ~ 90% of exchange for deuterium of the a-CH2
had occurred. The progress of isotope exchange of naphthalen-1-
ylacetic acid tert-butyl ester 2 was followed using a quench-based
method. Reactions in a volume of 12 mL were initiated by the
addition of 6 mL of stock solution of 2 (10 mM in CD3CN) to
a solution of potassium deuteroxide in D2O (6 mL), containing
internal standard, tetramethylammonium deuteriosulfate. The
progress of isotope exchange was determined by withdrawal of
800 mL aliquots of the reaction mixtures, which were quenched
with a 2.5 M DCl solution to pD < 10. The samples were placed in
a sealed plastic bag containing calcium chloride and were stored at
-18 ◦C until they could be analyzed by 1H NMR spectroscopy. For
naphthyl esters 1 and 2, the final substrate and internal standard
Naphthalen-1-ylacetic acid tert-butyl ester 2. Thionyl chloride
(3.00 mL, 41.1 mmol) was added under argon to 1-naphthylacetic
acid 14 (4.50 g, 24.1 mmol, recrystallised from ethanol). The
resulting solution was stirred for 24 h at room temperature.
Excess thionyl chloride was then removed in vacuo to give the
acid chloride 15 as a brown liquid (4.90 g, 23.9 mmol) which
1
was used without further purification. H NMR dH (400 MHz;
CDCl3) 7.99–7.21 (m, 7H, aromatic-H), 4.62 (s, 2H, CH2). tert-
Butanol (1.15 g, 15.4 mmol) and pyridine (8.46 g, 115.4 mmol)
were added to a stirred solution of the acid chloride 15 (2.50 g,
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concentrations in the reaction solutions were 5.0 mM and 2.0 mM,
respectively.
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1H NMR spectra were acquired with 64 transients, a delay of
15 s between pulses, an acquisition time of 6 s and a pulse width
1
of 4.8 ms. H NMR spectral baselines were subjected to a first-
order drift correction before integration of the peak areas. The
7 J. A. Gerlt and P. C. Babbitt, Annu. Rev. Biochem., 2001, 70,
209.
estimated error in the observed pseudo-first-order rate constants
1
for exchange (kobs, s-1) is 10% based on the error of the H
8 M. E. Glasner, J. A. Gerlt and P. C. Babbitt, Curr. Opin. Chem. Biol.,
2006, 10, 492.
9 A. J. Kirby and F. O’Carroll, J. Chem. Soc., Perkin Trans. 2, 1994, 649.
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L. Ottavi, J. Phys. Org. Chem., 2005, 18, 101.
NMR measurement. Although the measurements of kobs are single
determinations, the calculated error in similar measurements for
other carbon acids performed by J. P. Richard et al.19,20 is 10%
for kobs and 0.5 units for the pKa.
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pKa determination
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13 The half-life for the deuterium exchange reaction of naphthyl ester 1 at
the lowest concentration of KOD employed in our experiments (0.025
M) was 64 h at 25 ◦C.
All UV-Vis spectrophotometric measurements were recorded
using
mostatted at 25
was used for all measurements.
a
Varian Cary 50 UV-Vis spectrophotometer ther-
0.1 ◦C. The same quartz cuvette (1 mL)
stock solution of 8-
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(N,N-dimethylaminonaphthalen-1-yl)acetic acid tert-butyl ester
1 (20.3 mM) was prepared in HLPC-grade acetonitrile. UV-Vis
spectra were acquired in 0.01–2 M HCl, phosphoric acid buffers,
phosphate buffers and 0.1 M KOH in 1 : 1 H2O : CH3CN. With
the exception of the 0.5, 1.0 and 2.0 M HCl solutions, the ionic
strength of all solutions was maintained at 0.1 M (KCl). In each
case, the buffer, HCl or KOH solution (1 mL) was added (via glass
bulb pipette) to the quartz cuvette and allowed to equilibrate at
25 0.1 ◦C for 10 min. The stock solution of 1 (15 mL) was then
added via a Hamilton syringe to the cuvette to give a final substrate
concentration of 0.3 mM. After mixing, UV-Vis absorbance scans
(220–600 nm) were obtained and a fixed wavelength absorbance
was subsequently recorded at 325 nm over 10 min. In all cases, the
absorbance values were observed to be constant during this time
period. The observed absorbance values at 325 nm were corrected
for the background absorbance due to buffer, HCl or KOH at
the same wavelength. The pH of each solution was determined at
25 ◦C using a MeterLabTM PHM 290 pH-Stat Controller equipped
with a radiometer combination electrode.
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2005, 3, 3273.
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Acknowledgements
47, 1479.
31 F. G. Bordwell and H. E. Fried, J. Org. Chem., 1981, 46,
4327.
The authors thank the EPSRC (UK) for financial support for RJD
and SB, and UCD Dublin for financial support for CS. FH is an
ERC Starting Investigator. We gratefully thank the NMR staff
in Durham University for their ongoing help. We also thank our
referees for several helpful suggestions.
32 An additional prediction by extrapolation of the deprotonation rate
constants for ethyl acetate (using b = 1.09 measured with 3-substituted
quinuclidines)19 to an amine base of pKa 1 gives a rate contant kinter of
5 ¥ 10-14 M-1 s-1. A similar kinter value could be estimated for naphthyl
ester 1 which has comparable kDO and pKa values to ethyl acetate. For
kintra to be detectable using the NMR deuterium exchange method, this
value would have to be at least as large as the smallest first-order rate
constant for deuteroxide-catalyzed exchange of ester 1 (3 ¥ 10-6 s-1),
suggesting that the EM would have to be >108. This implies that the
EM would have to be unusually large – indeed larger than any other
observed from general acid/base catalysis2–4 – for an intramolecular
exchange process.
Notes and references
1 N. Backstrom, N. A. Burton and C. I. F. Watt, J. Phys. Org. Chem.,
2010, 23, 711.
2 A. J. Kirby, Adv. Phys. Org. Chem., 1980, 17, 183.
596 | Org. Biomol. Chem., 2012, 10, 590–596
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